1. Field of the Invention
The present invention relates to a spindle base of a height regulating device which can be used for a height regulated chair or a table, and more particularly to a spindle base which is formed in multi-layers by processing the inner and outer surfaces of a metal layer which constitutes the spindle base by using a nitration layer.
2. Description of the Prior Art
In general, a height regulating device used in a chair or a table includes a spindle 50 installed in a tube guide 55 installed in a cylindrical outer case 60 as shown in FIG. 1. The spindle 50 includes a spindle base 80, a cylinder 30 installed in the interior of the spindle base and filled with nitrogen gas in a predetermined pressure, a piston 11 which divides the cylinder to a first chamber 30a and a second chamber 30b, and a pipe holder 70 which has a gas opening/closing pin 20 for flowing the gas in the first chamber 30a and the second chamber 30b. 
The piston 11 is fixed to one end of the piston rod 10, and the other end of the piston rod 10 is fixed to an end of the outer case 60 by using a fixing means 40.
Since the interior pressure of the cylinder 30 is higher than the atmospheric pressure, if the gas opening/closing pin 20 is opened by pushing a button 33 so that the gas in the first chamber 30a and the second chamber 30b is moved through the gas inlet and outlet 16, the piston 11 is moved in the cylinder 30 by the pressure of the gas and the spindle is moved along the inner surface of the tube guide 55 to regulate the height of the height regulating device.
In the above-mentioned spindle, if the load applied from outside is not parallel to the axis, a reverse bending stress is generated on the contact surface of the spindle base 80 and the tube guide 55.
Especially, in the above-mentioned conventional spindle base 80, the spindle base of comprised of a single metal layer of aluminum and iron, the spindle base 80 becomes slim and a crack is generated in the spindle base 80 according to the generation of the reverse bending stress.
The problem of the spindle generated due to the reverse bending stress can be settled by making the thickness of the spindle base thick or using an expensive metal or forming a coating layer of a high hardness only on the outer surface of the spindle base.
However, in the above-mentioned methods, the fatigue stress of the metal cannot sufficiently satisfy or the manufacturing cost is increased.
The present invention has been made to solve the above-mentioned problem, and accordingly it is an object of the present invention to provide a spindle base which can improve the strength of the spindle base, the abrasion resistance, the corrosion resistance without enlarging the thickness of the spindle base by processing the surface of the spindle base with a nitration layer.
In order to achieve the above-mentioned object, the present invention provides a height regulating device gas which comprises a spindle base, a cylinder formed in the spindle base, a piston slidably moved in the cylinder, and a spindle having a piston rod having one end connected to the piston and in which the other end of the piston rod is fixed to one end of an outer case and the spindle is moved upward and downward through the outer case based on the piston rod and the outer case, wherein the spindle base is comprised of a metal and nitration layers are formed on the inner and outer surface of the metal.
The spindle base is comprised of iron. The thickness of the metal of the spindle base is 2.5 to 5 times of the sum of the thicknesses of the nitration layer of the inner surface and the nitration layer of the outer surface. The thicknesses of the nitration layer of the inner surface and the nitration layer of the outer surface are 200 to 400 xcexcm. The thickness of the nitration layer of the outer surface is equal to or greater than the thickness of the nitration layer of the outer surface.
The nitration layer is formed by stacking nitration layers having different hardness and the hardness of the outer side of the nitration layer is greater than the hardness of the inner side. The thickness of the outside of the nitration layer is 10 to 20 xcexcm.